1. Field of the Invention
The invention relates to internal combustion engines of the two-stroke type.
2. Description of the Prior Art
In a conventional two-stroke internal combustion engine, permanently open inlet and exhaust ports are usually provided at axially spaced locations in the cylinder wall, the exhaust port being closer to the cylinder head so that the piston exposes the exhaust and inlet ports sequentially as it moves towards bottom dead center permitting burnt gas to exit and air and fuel to enter the exhaust and inlet ports, respectively. During the return stroke of the piston, the inlet and subsequently the exhaust ports are closed as the piston rises away from bottom dead center.
In one type of two-stroke internal combustion engine, known as the crank-case scavenged type, the crank-case is hermetically sealed with the cylinder and provides a pressure chamber from which a fuel and air mixture is forced into the cylinder through a port opened by the piston moving through bottom dead center. This arrangement suffers from the disadvantage that crank-case oil necessary for lubrication is also carried into the cylinder with the fuel causing an undesirable emission of pollutants.
In another known type of two-stroke engine, the fan-scavenged type, a fan or supercharger blows a fresh gas-and-air mixture into the cylinder while the inlet port is uncovered so that residual burnt gas is flushed out of the exhaust port. The need for lubricating oil to be mixed with the fuel is avoided as a separate lubrication system can be provided.
Both types, however, suffer from the disadvantage that the exhaust and inlet are permanently open ports so that the timing depends entirely on the piston position which results in operation at reduced efficiency.
Furthermore, an undesirable limitation is imposed on the maximum compression ratio that can be obtained in practice arising, for example, from the exhaust port remaining uncovered for a substantial distance of travel of the piston during the early stages of the compression stroke.
In the latter type, problems can also occur, however, from residual burning gas or hot spots possibly causing pre-ignition of the incoming fuel.